Grid structure for color television tube



Jan. 17, 1956 J. coo 2,731,582

GRID STRUCTURE FOR COLOR TELEVISION TUBE Filed March 23, 1955 3Sheets-Sheet 1 Uwwm/wi 111mm: w/

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GRID STRUCTURE FOR COLOR TELEVISION TUBE Filed March 23, 1953 3Sheets-Sheet 2 CONDUCT/V5 COAT/N6 24 0M MODE 8.4!?

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GRID STRUCTURE FOR COLOR TELEVISION TUBE Filed March 23, 1953 3SheetsSheet 3 N0 8M5 0A) MODE am? 1 Smoow REG/0H -26 51.567704! SIIHDOWSCAM/W416 PnrrERu u/m/ SHORT 41005541? (Pas/7W5 8M5) y M A smmumaPflTTER/U WITH TALL 4/005 519/? /P05/7'/V ems) INVENTOR. Les/1'6 J CookGRID STRUCTURE FOR COLOR TELEVISION TUBE Leslie J. Cook, Lafayette,Calif., assignor to Chromatic Television Laboratories, Inc., New York,N. L, a corporation of California Application March 23, 1953, Serial No.343,887 9 Claims. (Cl. 315-14) The present invention relates tocathode-ray tubes of the type adapted to effect thereconstitution ofpolychrome images. More particularly the invention relates tocathode-ray tubes having a grid of coplanar parallel wires positionedadjacent to a striped phosphor screen, or target electrode, and to meansfor enhancing the color fidelity of the image reproduced on this screenby inhibiting any tendency the wires of the grid may have to vibrate asa result of the cyclicapplication of electrical potentials thereto.

Cathode-ray tubes constructed with a grid of parallel wires locatedadjacent to a striped phosphor screen are already known in the art andserveto focus the beam electrons into a pattern of thin parallel linesregistered with the phosphor strips of the screen. The PDF(post-defiection-focusing) type of cathode-ray tube operation has beenset forth by Ernest 0. Lawrence, in various of his copending UnitedStates patent applications, such as Serial No. 219,213, filed April 4,1951, issued October 26, 1954, as Patent No. 2,692,532, and Serial No.234,190, filed June 29, 1951, issued June 21, 1955, as Patent No.2,711,493.

In order to facilitate an understanding of the principles of the presentinvention, a brief description of one such form of single-gun PDF tubewill now be given. This description should be construed as exemplaryrather than limiting, since it will be seen that the invention isobviously applicable to tubes constructed along diiferent lines. Ingeneral, however, the tube may incorporate a .screen, or targetelectrode, made up of a relatively large number of very narrow phosphorstrips laid down in a predetermined sequence to develop, when impartedby a scanning cathode-ray beam light in a selected chromatic sequence,such as red, green, blue, green, red, green, etc. The phosphors are thenaluminized, or the screen in some other manner is provided with anelectrically conductive coating. For convenience of reference thephosphor strips will be herein identified by a reference to the color oflight developed, such as the red phosphor. Similarly the grid wires willbe identified by the fictitious reference to the color of light which isdeveloped when the grid wire is in its most positive state relative tothe electron source, such, for instance, as ared grid wire when the gridWire potential is such that the electrons from the source impact aphosphor strip to develop red light.

A grid assembly is located adjacent to such phosphor screen. The gridmay be made up of parallel coplanar wires, and so related to the.phosphor strips that, in an electron-optical sense, there is a wirealigned with each blue strip, and similarly a wire aligned with each redstrip. The red wires are connected to a common terminal, while the bluewires are similarly joined together electrically.

Between the plane of the wire grid assembly and the conductive coatingon the phosphor screen, there may be established a difierencepfpotentialofsuch magnitude and polarity as to create a series of convergingcylindrical lenses for the electrons in the scanning beam. In otherwords, the beam electrons entering between any pair of grid wires inpassing to the target are brought to a line focus at the plane of thetarget or screen, this line structure having no necessary directgeometrical relationship to the path covered by the scanning beam intracing the lines of the image raster.

It will now be appreciated that, as the beam electrons travel from theelectron gun they may be focused by the above-described lens structureinto a series of lines parallel to the phosphor strips. if there is azero potential difference between the red and fblue terminals of thewire grid, then these lines formed by the beam electrons may be causedto lie within the boundaries of the green strips. lf'the wiresassociated with the red strips are made positive relative to the wireselectronoptically related to the blue strips, the beam electrons will bedeflected, and the thin lines will now lie within the boundaries of thered strips. Similarly, the electrons will strike the blue strips whenthe wires associated with such strips are relatively positive withrespect to the red? wires. Difierent component colors are thus displayedaccording to the potential difference (if any) existing between the twosections of the grid wire assembly.

Accordingly,.color control in a cathode-ray tube having a grid assemblyof the above nature (whether used for PDF or not) is brought about by acyclic change in the potentials applied to selected wires of the grid,with the electrostatic charges on the wires exerting attractive forcesthe magnitude of which varies as a function of this potential change. Ifthe forces thus exerted vary at a frequency close to the naturalresonant frequency of the wires, the latter will vibrate, resulting inan oscillation of the line pattern on the phosphor screen. If themagnitude of this oscillation is sufficiently great, color contaminationand/or electrical shorting can occur. Even with relatively little wirevibration the electron beam may be defocused, and image reproductionseriously impaired.

The above problem has been recognized. One solution is proposed in acopending United States patent applica tion of James T. Vale, Serial No.252,664, filed October 23, 1951, and assigned to the same assignee asthe present application. In the Vale disclosure, the vibration of such awire grid is damped, and one preferred arrange ment for carrying outthis objective includes an insulating cord or strand (which may be aceramic thread) wound, or passed in-and-out, between the parallelconductors in a region near the wire frame or support.

The above-mentioned Vale method of reducing the amplitude of grid wirevibration by bringing into contact with the wires a mechanically lossymaterial is efiective for many purposes where some residual vibrationcanbe tolerated. However, friction between the wires and the damping membernecessitates the use of materials able to resist this abrasive action.

An alternative method of substantially eliminating grid wire vibrationis disclosed in the present application. This embodies the principlethat the frequency of vibration of a supported wire is a function of thedistance between its supports. Hence, if a wire of given length (havinga certain natural resonant frequency) is rigidly held at one or morepoints intermediate its ends,

' then the wire segments between the constraining points will havehigher resonant frequencies corresponding to their shorter lengths. Ifthese increased resonant frequencies are sufficiently higher than thedriving frequency (i. e., the rate of change of potentials applied tothe wires) then the tendency of the wire segments to vibrate will bematerially decreased.

One object of the present invention, therefore, is to provide animproved form of cathode-ray tube suitable for the reconstitution ofpolychrome images.

A further object of the invention is to substantially completelyovercome, in a polychrome cathode-ray tube having a color-control gridstructure of coplanar parallel wires, any tendency toward vibration thewires of the grid structure may possess when color-changing potentialsare cyclically applied thereto.

-An additional object of the invention is to provide, in a polychromecathode-ray tube having a color-control grid structure of coplanarparallel wires, 2. method and means for reducing, or substantiallyeliminating, any electron shadow which might otherwise result from theemployment of one or more vibration-reducing elements in conjunctionwith the wire grid.

Other objects and advantages will be apparent from the followingdescription of a preferred form of the invention and from the drawings,in which:

Figure 1 is a diagrammatic representation of one form of cathode-raytube in which the present invention may be incorporated;

Figure 2 is a perspective view of a number of the grid wires andphosphor strips of Figure 1, showing one pos sible relationshiptherebetween;

Figure 3 is a perspective view of a preferred type of vibration-reducingmember constructed in accordance with the present invention;

Figure 4(a) is a cross-sectional view of the vibrationreducing member ofFigure 3, showing in addition the electron shadow region which wouldnormally appear on the target electrode in the absence of the conductivecoating of the present invention;

Figure 4(b) is a representation of a scanning raster such as would beproduced when utilizing the structure of Figure 4(a);

Figures 5(a) and 6(a) show the result of adding a conductive coating tothe vibration-reducing member of Figure 4(a) so as substantially toeliminate the electron shadow region of the latter figure; and

Figures 5 (b) and 6(1)) are representations of scanning tasters such aswould be produced when utilizing the arrangements of Figures 5(a) and6(a), respectively.

In Figure 1 of the drawings there is generally identified by thereference numeral 10 one type of cathode-ray tube in which the presentinvention may be incorporated. Thistube 10 includes the usual componentsfor developing a beam of electrons and for deflecting this electron beamin substantially mutually perpendicular directions so as to trace animage raster on the tube target electrode. Since these components arewell known in the television art, no detailed description of theiroperation is believed necessary.

Adjacent to the end wall of the tube 19, and positioned to be impingedby the electron beam 12, is a target electrode 14. A grid 16 of parallelwires lies in a plane adjacent to the plane of such target electrode.

The target electrode 14 may assume a number of forms, one of whichconsists of a plurality of phosphor strips 15 deposited side-by-side ona thin plate of glass or other transparent material. These phosphorstrips 15 have the property of fluorescing in different colors, and, asan illustration, the target electrode may include strips respectivelyfluorescing in the three primary colors, red, green and blue. Anyselected colors, however, may obviously be represented by the stripsaccording to the particular phosphor compositions employed.

As best shown in Figure 2, the phosphor strips are laid down in apredetermined chromatic sequence, with a green strip between each redand blue strip. The order in which the strips appear, however, forms nopart of the present invention. Moreover, a discussion of phosphor screenconstructions folrns part of copending application Serial No. 234,190referred to above, and hence no additional details are believednecessary in the present dethe grid wires, diameter of the grid wireframe, etc.

scription except to indicate that the screen is given a thin coating ofaluminum, or other electrically conductive electron-permeable material,which may be deposited or placed upon the phosphors in any suitablemanner.

As above stated, the grid 16 consists of a plurality of parallel wireslying in a plane adjacent to the plane of the phosphor strip 15. Onepossible relationship of the grid wires and phosphor strips isillustrated in perspective in Figure 2. It will be noted that therelative dimensions and spacings of the components in this figure areintentionally distorted. However, each pair of wires, in anelectron-optical sense, subtends strip areas constituting one colorcycle.

In a preferred form of tube design, a potential is applied to theconductive coating on the phosphor screen which is different from theaverage, or D.-C., potential of the wires of the grid. This gives riseto a plurality of cylindrical electrostatic lenses, which serve to focusthe electrons in the scanning beam into a series of fine linesregistered with the phosphor strips. Thus, in effect, the structure 16performs the dual function of a lens-grid and a color-control component.However, the invention is obviously applicable to cases where the grid16 serves as a color-changing device alone, as will appear below.

It has been stated above that one of the principal features of thepresent invention consists in substantially completely overcoming anytendency toward vibration the wires of a color-control grid assembly ofthe class described may possess by constraining such wires at one ormore points between their ends, and thus increasing their naturalresonant frequency so that it is distinct from the rate at which thecolor-control potentials are varied.

Fundamentally, the above objective is achieved by utilizing any suitabletype of solid constraining means. One method comprises casting a singlefilament of cementitious material (such for example as Sauereisencement) on the center line of the grid structure and at right angles tothe wires, thus embedding the latter in a rigid matrix. In practice,this method is not completely satisfactory as the cement tends to shrinkwhile curing, thus displacing the wires. On the other hand, if thelatter are held with a restraining fixture, the cement is prone tocrack.

Another approach consists in alternately interleaving a pair of glassthreads in-and-out between the grid wires and along the center line ofthe grid structure. The thread is then impregnated with some suchmaterial as potassium silicate to make a rigid assembly. This expedientmay be acceptable for tube designs where an extremely precise grid unitis not required, but in a majority of cases the resulting wiredisplacement precludes its use.

The method which has proven to be the most satisfactory in practice isbased upon the use of one or more rigid strips, or bars, of glass orother ceramic insulating material. One form is illustrated in Figure 3,where such a bar, designated by the reference numeral 18, is provided onone edge with a series of V-shaped notches 20 on the same centers as thewires of the grid 16. The sides of the bar 18, as shown in the drawing,are fiat, and the bar should preferably be relatively wide with respectto its thickness /2" width and .032 thickness is satisfactory in manycases) although its exact dimensions are not critical and will vary inaccordance with the length of The plane of the bar 18 should be orientedto include the center of deflection of the electron beam. It isimportant in the construction of a tube of the character described thatthe likelihood of a shadow on the target be reduced to a minimalpossibility. To this end the node bars, when positioned relative to thegrid Wires, should be secured and anchored in such a manner that thesides are alined with and become parallel to the electron trajectorytoward the target.

The bar 18 is scout-that the wires 16 (when in position as shown inFigure 3) lie at the bottom of each V-shaped notch. In practice, thesenotches 20 are each filled with a cementitious binder 22 (such asSauereisen cement), and the bar is placed against the grid Wires so thateach wire is effectively imbedded in a ball of binding material, which,after drying, locks the wires inplace in the same relative position ineach notch. The ball of binding material is shown in Figure 3 asextending slightly over the edge of the bar 18. For the stated reasonsit will be appreciated that the bar 18 may appropriately be termed anode bar, since it creates a node, or point of zero vibration, for eachof the grid wires 16. Although some shrinkage of the cementitious binder22 may occur as it dries, there is no appreciable displacement of thegrid wires, since the discontinuity of the binding areas causes theresultant force developed in any one direction to be negligibly small.In other words, the single filament of cementitious material formerlyemployed is effectively broken down into a plurality of small bits, theshrinkage of which can be disregarded.

With the node bars located as described and with the cementitiousbinders serving to secure the node bars and the grid wires to each otherit will be apparent that the grid wires thus form the support within thetube for the node bars. In the manufacturing process the preciselocation of each node bar with respect to the grid wires is determinedin accordance with the geometry of the tube type being constructed, itbeing apparent that with a long tube, where the electron path betweenthe electron gun and the target is long as compared to any targetdimension, there will be but small angular change in the supportposition of one node bar relative to the other. However, for shortertubes where the electron path from the electron gun to the target is aconsiderably smaller'multiple of any target dimension the angular changein the plane of each node bar with respect to each other node barbecomes more pronounced in order to maintain surface parallelism withrespect to the electron beam trajectory. in either instance, it shouldbe borne 'in mind that it is desirable that as few of the beam electronsas possible should be intercepted by the node bars in order that thealready mentioned shadow effect be overcome.

Since the node bar 18 lies between the electron gun of tube and thetarget electrode 14, it possesses two properties which are common to allstructures positioned so as to intercept a scanning beam. Firstly, whenthe structure is a dielectric, it will acquire an electro-static chargeif there is an unbalance between the number of impinging beam electronsand the number of electrons lost through secondary emission. Secondly,being in the beam path, the structure will tend to cast an electronshadow on the target electrode.

The first of the above problems is solved by rendering at least aportion of the surface of the node bar electrically conductive, and thenconnecting this conductive surface to a point of fixed potential. Onemethod of doing this is to paint at least a portion of the bar surfacenearest the electron gun with a thin film of material such as aquadag.The conductive film 24 thus created is electrically connected to anyconstant potential point which is positive in polarity, and of theproper magnitude, with respect to the average, or D.-C. potential of thegrid wires 16. The establishment of such relative potentials between theconductive coating 24 of the bar 18 and the grid wires 16 is more orless schematically illustrated in Figure 3 as comprising a battery 25,although it will be appreciated that such an element would not actuallybe used in practice. The establishment of an optimum potential for thenode bar as will later be pointed out in more detail, brings about asolution of the second of the two problems previously set forth-namely,that of an electron shadow on the target electrode.

Let us first assume that the conductive coating 24 is omitted, and thebar 18 employed without this feature. Reference to Figure 4 (a) showsthat the bar 18 inter- 6 cepts certain of the scanning beam electrons(having the trajectories 24) and thus creates a shadow region 26 underthe bar. An actual raster which might be produced (the scanning linesbeing at an angle ofapproximately both to the grid wires 16 and to thenode bar 18) is illustrated in Figure 4 (b). The shadow region isobviously quite pronounced, and would prove definitely objectionable toan observer.

In Figure 5 (a) is shown the effect of providing the node bar 18 withthe conductive coating 24 described above, and then connecting thiscoating 24 to any suitable fixed potential point which is relativelypositive with respect to the average potential of the grid wires 16.From the configuration of the electric field lines 28 surrounding thecoating 24, it will be seen that many scanning beam electrons (whichtend to cross these field lines at right angles thereto) will be bentinwardly near the lower portion of the bar 18 and thus tend to fill upthe region 26 formerly in shadow. In other words, a majority of theelectrons which pass in the immediate neighborhood 7 of the bar, butwhich do not impinge the conductive coating 24, will be deflectedintothe region 26. Ohviously, these electrons thus deflected wouldotherwise have impinged the target electrode 14 outside of the shadowregion, so that actually the latter is filled at the expense of theelectron flux immediately adjacent to it. A certain amount of imagedistortion may result, but this can be minimized in a manner now to bebrought out.

The extent of the region from which the filling electrons come isdependent upon the vertical height of the bar 18 (its width aspreviously considered in connec tion with Figure 3). If this verticaldimension is considerable, as in Figure 5 (a) for example, then theshadow region 26 is filled in with electrons from a substantial distanceon each side of the bar 18. The resulting distortion is fairly smoothand flowing, as illustrated in Figure 5 (b).

When the node bar 18 is shorter, as shown in Figure 6 (a), the electriclines of force differ considerably in configuration from those shown inFigure 5 (a). The electrons filling up the shadow region 26 now comeonly from immediately adjacent the bar, and there is an abruptstretching of the resulting image (as shown in Figure 6 (11)) which maybe objectionable to an observer. Accordingly, the height of the bar 18should be optimized (for each tube design in which such a bar isemployed) so as to produce minimum apparent video distortion.Experiments seem to show that best results are obtained when the widthof the shadow is reduced to a very small value (such as .005",for'example) rather than completely eliminated. However, this is amatter of individual preference.

In one design of a cathode-ray tube with which the node bar of thepresent invention may be employed, the wire grid 16 is biased negativelywith respect to the metal shell of the tube. Accordingly, in such a caseit is not normally necessary to provide a separate electrical lead forthe node bar, and it may be connected within the tube directly to thismetal shell. Since the potential difference between the conductivecoating on the node bar, on one hand, and the average potential of thegrid wires, on the other, is what constitutes the operating condition ofthese elements, the latter may be adjusted by varying the relativelynegative grid voltage while the node bar potential remains constant.

It should be understood that use in the present application of theexpression adjacent to in describing the spatial relationship of thewire grid and target electrode is to be interpreted as covering acondition wherein the wire grid lies in a plane slightly spaced apartfrom the plane of the target electrode, and is not to be confused with acase where these two members are contiguous, or actually in physicalcontact with one another.

Having thus described the invention, what is claimed is:

1. In a cathode-ray tube designed for the reconstitution of polychromeimages, a color-control grid structure of parallel wife's-lying in aplane adjacent to the surface of the target electrede scanned by theelectron bear'n'of said cathode-ray tube, and means for'inateriallyreducing any tendency the parallel grid wires of said color-controlstructure may have to vibrate when ditferent color-control potentialsare cyclically applied thereto, said vribationreducing means includinginsulating means for effectively dividing each grid wire into aplurality of sections each of which has a higher natural resonantfrequency than that possessed by the wire prior to its etiectivedivision.

2. A node bar for a cathode-ray tube designed for the reconstitution ofpolychrome images and having a colorcontrol grid structure of parallelwires lying in a plane adjacent to the surface of the target electrodescanned by the electron beam of said cathode-ray tube, said node barcomprising a strip of insulating material lying substantially in theplane of said grid wires and extending transversely thereto, and bindingmeans rigidly securing said grid Wires to said insulating strip atsubstantially equally-spaced points ther'ealong, thereby substantiallyto preclude any tendency the said grid wires would otherwise possess tovibrate when different color-control potentials are cyclically appliedthereto.

3. A node bar according to claim 2, in which said strip of insulatingmaterial is provided with a series of equallyspaced notches along onesurface thereof, said grid wires being respectively receivable in saidnotches.

4. A node bar according to claim 3, in which each of the saidequally-spaced notches in said insulating strip is of substantiallyV-shape and in which each wire is positioned generally at the apex ofthe V and wherein the said binding means comprises a mass ofcementitious material surrounding the wire at its contact with the notchand contacting the node bar surface so that said grid wires arerespectively embedded in a cementitious mass and secured to the node barthereby.

5. A node bar for a cathode-ray tube designed for the reconstitution ofpolychrome images and having a colorcontrol grid structure of parallelwires lying in a plane adjacent to the surface of the target electrodescanned by the electron beam of said cathode-ray tube, said node barcomprising a rigid strip of insulating material extending substantiallytransversely to the direction of said grid wires, means for rigidlysecuring said grid wires to said insulating strip at such points alongthe latter as to maintain the parallel relationship of said grid Wires,and means for rendering at least a portion of the surface of saidinsulating strip electrically conductive.

6. A node bar according to claim 5, further comprising means-for'connecting the conductive surface portion of said insulating strip to apoint of fixed potential.

7. The combination according to claim 6 in which the said grid wireshave an average operating potential, further comprising means formaintaining a selected difference between the fixed potential of theconducting surface portion of the insulating strip', on one hand, andthe average operating potential of the said grid wires, on the other,the former being relatively positive in polarity with respect to thelatter.

8. A node bar for a cathode-ray tube designed for the reconstitution ofpolychrome images and having a colorcontrol grid structure of parallelwires lying in a plane adjacent to the surface of the target electrodescanned by the electron beam of said cathode-ray tube, said node barcomprising an elongated member of insulating material disposed generallytransversely to the direction of the parallel grid wires, means forsecuring said grid wires to said elongated member at such pointstherealong as to maintain the parallel relationship of said grid wires,and further means for substantially completely eliminating the electronshadow which would normally be formed on the surface of the said targetelectrode due to the presence of said elongated member in the path ofthe electron scanning beam of said cathode-ray tube.

9. The combination of claim 8 in which said grid wires have an averageoperating potential, and in which said means for substantiallycompletely eliminating said electron shadow includes means for renderingat least a portion of the surface of said elongated member electricallyconductive, together with means for establishing a predetermined valueand polarity of bias between such conductive surface portion of saidelongated member and the average operating potential of said grid wires,thereby to deflectelectrons from said scanning beam onto that portion ofthe surface of said target electrode where said electron shadow wouldnormally appear due to the presence of said elongated member.

References Cited in the file of this patent UNITED STATES PATENTS2,416,056 Kallmann Feb. 18, 1947 2,446,791 Schroeder Aug. 10, 19482,461,515 Bronwell Feb. 15, 1949 2,568,448 Hansen Sept. 18, 19512,590,764 Forgue Mar. 25, 1952

